The present invention relates to raster printer controllers for controlling print engines, and more particularly, to a computerized method operable on a raster printer controller for reducing pixel density in high pixel density areas of bitmaps before dispatching such bitmaps to the print engines.
Certain printing engines or marking engines utilize a non-impact printing technology known as magnetography. In magnetography a revolving metal drum is selectively magnetized by an array of small electromagnets to create a latent magnetic image on the drum. The drum is then exposed to magnetic toner particles which are retained by the magnetized image areas of the drum. The image is then transferred onto a substrate such as paper and subsequently fused thereto. Commercially available marking engines utilizing the magnetographic technology include the Nipson M700 and 7000 Series printers. A more detailed description of the magnetography process can be found in Nipson""s web page, xe2x80x9chttp://www.nipson.com/magnetog.htmxe2x80x9d
Single color (i.e., black and white) versions of such magnetographic printers receive bitmaps from printer controllers, and in turn, transfer the bitmaps to the paper utilizing the magnetographic process described above. Such bitmaps are generally two-dimensional arrays of binary digits, corresponding to two-dimensional arrays of pixels to be transferred to the printed page. A positive valued binary digit (xe2x80x9c1xe2x80x9d) in the bitmap corresponds to a solid-colored pixel in the printed image while a null valued binary digit (xe2x80x9c0xe2x80x9d) in the bitmap corresponds to a blank or clear pixel in the printed image.
One known problem with the magnetographic process is that the intensity of the magnetic domains that are created on the imaging drum is non-linear. Extremely dense areas in the image (areas in the image having a multitude of packed-together solid-color pixels) will often cause the magnetic toner to bleed over into surrounding, less dense, areas. Narrow bands of blank pixels surrounded by the dense areas of solid-color pixels are particularly sensitive to this bleed over effect.
Accordingly, there is a need for a means to reduce the density of solid-color pixels in high-density solid-color areas within a bitmap, prior to printing the image represented by the bitmap, to compensate for the non-linear behavior of the magnetographic marking engine.
For the purposes of this disclosure, the following terms are defined as follows:
Pixel Map: a two-dimensional arrangement of elements, corresponding to a two-dimensional arrangement of pixels making up a printed image. Of course, a bitmap is an example of a pixel map.
Positive Valued Pixel Element: an element of a pixel map corresponding to a solid-colored pixel in the printed image. Typically represented by a binary xe2x80x9c1xe2x80x9d in a bitmap.
Null Valued Pixel Element: an element of a pixel map corresponding to a blank or clear pixel in the printed image. Typically represented by as a binary xe2x80x9c0xe2x80x9d in a bitmap.
The present invention provides a computerized method which compensates for the non-linear behavior of the magnetographic marking engines by reducing the density of solid-color pixels in high-density solid-color areas within an image. The method may be performed by the printer controller or by the marking engine itself, resulting in improved image quality. Preferably, the computerized method also preserves the less dense portions of the image.
In one embodiment of the present invention, a computerized method for controlling the density of pixels deposited by print a engine includes the steps of: (a) obtaining a two-dimensional pixel map of an image to be printed; (b) selecting a first positive valued pixel element in the pixel map; (c) changing the first pixel element to a null value if a predetermined number of pixel elements in the pixel map immediately adjacent to the first pixel element have a positive value; and (d) dispatching the pixel map to the print engine.
In a preferred embodiment of this method, the predetermined number of pixel elements in the pixel map immediately adjacent to the first pixel element having a positive value must equal four (corresponding to all of the immediately adjacent pixel elements). Preferably this method is performed for each pixel element in the pixel map prior to dispatching the pixel map to the print engine. Furthermore, in various embodiments of the present invention, borders of high density pixel areas need not be subjected to the above test.
In another embodiment of the present invention a computerized method for controlling the density of pixels deposited by a print engine includes the steps of: (a) obtaining a two-dimensional pixel map of an image to be printed; (b) selecting a first positive valued pixel element in the pixel map; (c) assigning positive value pixel elements in the pixel map immediately adjacent to the first pixel element a first weight; (d) assigning positive valued pixel elements in the pixel map diagonally adjacent to the first pixel element a second weight; (e) adding the assigned first and second weights together to produce a total weight; (f) changing the first pixel element to a null value if the total weight is equal to or greater than a predetermined threshold; and (g) dispatching the pixel map to the print engine. In a preferred embodiment, the weight to be applied to immediately adjacent positive pixel elements equals xe2x80x9c5,xe2x80x9d the weight applied to diagonally adjacent positive pixel elements equals xe2x80x9c1,xe2x80x9d and the threshold value equals either xe2x80x9c16xe2x80x9d or xe2x80x9c17.xe2x80x9d
Accordingly, it is an object of the present invention to provide a computerized method for reducing the density of positive valued pixels in high density areas prior to transmitting the pixel map to the marking engine. It is a further object of the present invention to preserve less dense portions of the image.
It is a further object of the present invention to provide a computerized method for controlling the density of pixels deposited by a print engine that comprises the steps of: (a) obtaining a two-dimensional pixel map of an image to be printed; (b) comparing the value of a first pixel element in the pixel map with values of pixel elements adjacent to the first pixel element in the pixel map; (c) changing the value of the first pixel element responsive to a determination, in the comparing step, that a predetermined number of the pixel elements adjacent to the first pixel element have the same value as the first pixel element; and (d) dispatching the pixel map to the print engine.
It is a further object of the present invention that the above changing step be performed responsive to a determination, in the comparing step, that all pixel elements immediately adjacent to the first pixel element have the same value as the first pixel element.
It is a further object of the present invention that the comparing step includes the steps of assigning pixel elements immediately adjacent to the first pixel element and having the same value as the first pixel element a first weight, assigning pixel elements diagonally adjacent to the first pixel element and having the same value as the first pixel element a second weight different than the first weight, adding the weights for the immediately adjacent and diagonally adjacent pixel elements to produce a total weight, and testing the total weight against a predetermined threshold; and where the changing steps include the step of changing the value of the first pixel element responsive to a determination, in the comparing step, that the total weight is equal to or greater than the predetermined threshold.
It is also an object of the present invention to provide a software program, stored on a memory device, configured to control a printer controller or a marking engine to perform the steps of: (a) obtaining a two-dimensional pixel map of an image to be printed; (b) preparing a value of a first pixel element in the pixel map with values of pixel elements adjacent to the first pixel element in the pixel map; and (c) changing the value of the first pixel element responsive to a determination, in the comparing step, that a predetermined number of pixel elements adjacent to the first pixel element have the same value as a first pixel element.
These and other objects and advantages of the present invention will be apparent from the following description, the attached drawings, and the appended claims.